Polyoxometalate-Modified Metal and Metal Oxides for Multifunctional Catalysis 公开

Abstract

Polyoxometalate-modified metal and metal oxide materials have been applied in a wide range of fields. This dissertation focuses on the synthesis of the polyoxometalate-modified metal and metal oxides and studies their activities for photocatalysis and electrocatalysis. Chapter 2 describes the decatungstate-stabilized platinum nanoparticles (W10-PtNPs) for photochemical dehydrogenation of alkanes at room temperature. The W10-PtNPs (~ 20 nm) catalyze the reaction of alkane to produce alkene and hydrogen in acetonitrile under UV light with high selectivity and quantum efficiency. Several characterization methods confirm the integrity of decatungstates that absorb on the surface of platinum nanoparticles during the catalysis. The calculation model of a (Na-3MeCN)4W10O32/Pt19 complex reveals the strong interaction between the Pt and decatungstates. Chapter 3 examines polyoxometalate-stabilized gold nanoparticles for electrochemical CO2 reduction. Several Keggin-type POMs (PW12O40^3- and SiW12O40^4-) are capable of protecting Au nanoparticles in aqueous solution by forming POM-AuNPs, which are centrifuged and deposited on the electrode for electrochemical CO2 reduction in acetonitrile solution with 0.5% (v/v) H2O. However, these POM-AuNPs only produce CO and show no obvious advantages compared to citrate-AuNPs. Chapter 4 reports a POM immobilization method by anchoring APS ligand on metal oxide light absorber that successfully fabricates a Co9POM functionalized TiO2 photoelectrode with three-fold photocurrent enhancement compared to bare TiO2 in acid condition. We demonstrate the hybrid photoanode can operate for up to 5 hours with maintained Co9POM structural integrity. To the best of our knowledge, this is the first application of functionalization of efficient OER catalyst Co9POM on photoanode that achieves efficient water photo oxidation in acid condition. Further mechanistic studies conclude that the enhancement of photocurrent is mainly due to Co9POM acting as a fast hole collector and active catalytic center rather than surface states passivation. TA spectroscopies further verify the fast photogenerated hole transfer from TiO2 to Co9POM at ps timescale. The highly charged surface by Co9POM also modifies the TiO2 band edge, enabling a suitable surface electric field to separate the surface photogenerated charge carrier. Chapter 5 describes the incorporation of PVxMo12-xO40^(3+x)- (x = 1-3) and transition-metal-substituted polytungstates, XPW11 (X = V, Co, Zn and Co) into the pores of HKUST-1. Studies show that the reactivity synergism exists between Cu(II) in the nodes of HKUST-1 and PVxMo12-xO40^(3+x)- (x = 1-3) but not for the corresponding transition-metal-substituted polytungstates XPW11 (X = V, Co, Zn and Co). In addition, synergism in activity also leads to synergism in stability by fast electron transfer between the POM and the MOF framework. For the XPW11@HKUST, the Cu(II) nodes decompose to Cu(I) structures discovered by X-ray photoelectron spectroscopy (XPS).

Table of Contents

Chapter 1: Introduction to Polyoxometalate-Modified Metal and Metal Oxides for multifunctional catalysis 1

1.1 Overview of Polyoxometalates2

1.1.1 Polyoxometalate Structures 2

1.1.2 Polyoxometalate Redox Properties4

1.1.3 Mixed-Valence Polyoxometalates7

1.2 Polyoxometalate-Modified Metal Materials 9

1.2.1 Synthesis of POM-modified metal nanoparticles10

1.2.2 Applications POM-modified metal nanoparticles 12

1.3 Polyoxometalate-Modified Metal Oxide Materials15

1.3.1 POM-modified p-type photocathodes16

1.3.2 POM-modified n-type photoanodes 17

1.4 Polyoxometalate-Immobilized Metal Organic Frameworks19

1.5 Scope of Current Work 22

1.6 References 23

Chapter 2: Polyoxometalate-Stabilized Platinum Nanoparticles for Photochemical Alkane Dehydrogenation 42

2.1 Introduction 43

2.2 Experimental 45

2.2.1 General Materials and Methods 45

2.2.2 Synthesis of Na4W10O32-stabilized Pt nanoparticles (W10-PtNPs)46

2.2.3 Photocatalytic alkane dehydrogenation 46

2.3 Results and Discussion47

2.3.1 Synthesis and characterization of W10-PtNPs47

2.3.2 Reactivity and stability of W10-PtNPs55

2.3.3 Transient absorption spectroscopy60

2.3.4 Calculations60

2.4 Conclusions 66

2.5 References 67

Chapter 3: Polyoxometalate-Stabilized Gold Nanoparticles for Electrochemical Carbon Dioxide Reduction72

3.1 Introduction 73

3.2 Experimental 81

3.2.1 General Materials and Methods 81

3.2.2 Electrochemistry 82

3.3 Results and Discussion 82

3.3.1 Synthesis and characterization of PW12-AuNPs/Citrate-AuNPs 82

3.3.2 Reactivity of PW12-AuNPs/Citrate-AuNPs91

3.4 Conclusions 94

3.5 References 95

Chapter 4: Co-Polyoxometalate-TiO2 Photoelectrode for Efficient Oxygen Evolution Reaction in Acidic Media103

4.1 Introduction 104

4.2 Experimental 111

4.2.1 General Materials and Methods 111

4.2.2 Preparation of POM/Co-Pi modified TiO2 Photoelectrodes111

4.2.3 Photoelectrochemical Studies113

4.2.4 Transient Absorption (TA) Spectroscopy Studies114

4.3 Results and Discussion 116

4.3.1 Characterization of TiO2 and TiO2-Co9POM photoelectrodes 116

4.3.2 Photoelectrochemical Oxygen Evolution by TiO2-Co9POM 120

4.3.3 Transient Absorption Spectroscopies of TiO2-Co9POM126

4.4 Conclusion 132

4.5 References 132

Chapter 5: Polyoxometalate-Incorporated Metal−Organic Frameworks for Aerobic Thiol Oxidation143

5.1 Introduction 144

5.2 Experimental 146

5.2.1 General Materials and Methods 146

5.2.2 Synthesis of POM@HKUST147

5.2.3 Thiol (RSH) oxidation148

5.3 Results and Discussion 148

5.3.1 POM@MOF Materials and Characterization 148

5.3.2 POM Leaching and Solvent Selection151

5.4 Conclusion 152

5.5 References 153

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School	Laney Graduate School
Department	Chemistry
Degree	Ph.D.
Submission	Dissertation
Language	English
Research Field	Chemistry, Inorganic
关键词	Polyoxometalates Metal Nanoparticles Catalysis
Committee Chair / Thesis Advisor	Craig Hill, Emory University Brian Dyer, Emory University Cora MacBeth, Emory University

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